1. Field of the Invention
The present invention relates to control of a downlink channel decoder in an Institute of Electrical and Electronics Engineers (IEEE) 802.16e Broadband Wireless Access (BWA) communication system. More particularly, the present invention relates to an apparatus and method for avoiding decoding of unnecessary Forward Error Correction (FEC) blocks using Connection Identifier (CID) information in a Medium Access Control (MAC) header.
2. Description of the Related Art
An active study area for 4th Generation (4G) communication systems is the provisioning of services with diverse Quality of Service (QoS) requirements at or above 100 Mbps to users. Particularly, active research is being conducted on providing high-speed service by ensuring mobility and QoS to a BWA communication system such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Such major examples are IEEE 802.16d and IEEE 802.16e. Worldwide Interoperability for Microwave Access (WiMAX) and Wireless Broadband (WiBro) adopts the IEEE 802.16d and 802.16e technologies.
The IEEE 802.16d and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) for physical channels in order to support a broadband transmission network. IEEE 802.16d considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SS's mobility to the IEEE 802.16d communication system.
FIG. 1 illustrates a frame structure in the IEEE 802.16e communication system.
Referring to FIG. 1, a frame is a basic unit for data transmission in the IEEE 802.16e communication system. The frame is 5 ms in duration and includes 42 OFDM symbols.
The IEEE 802.16e frame is divided into a DownLink (DL) subframe and an UpLink (UL) subframe. The frame is comprised of a preamble, a Frame Control Header (FCH), a DL-MAP, DL bursts, a UL control channel, and UL bursts. A burst is a basic data transmission unit. The frame has a plurality of slots each slot being a basic unit for frequency resource allocation. One slot has 48 subcarriers. An arbitrary number of slots can be allocated to one burst.
In the IEEE 802.16e communication system, radio resources are allocated in slots. As stated above, a signal or data transmission unit using radio resources is referred to as a burst, for which a Modulation and Coding Scheme (MCS) is defined by a Downlink Interval Usage Code (DIUC) in the DL-MAP. The channel coding can be tail-biting convolutional coding or Convolutional Turbo Coding (CTC). Generally, the CTC is adopted for encoding data bursts, due to its excellent error correction performance.
When the CTC is used in the DIUC-based burst definition scheme, a maximum FEC unit for a CTC (i.e. the number of information bits included in a CTC codeword) has 480 bits. A FEC block size is determined for each MCS. If a burst size is larger than the maximum FEC block size, an information sequence is subject to fragmentation, encoding, and concatenation according to the standard.
FIG. 2 illustrates burst fragmentation and concatenation in the IEEE 802.16e communication system.
Referring to FIG. 2, a maximum FEC block size is expressed as the number of slots. Given a burst with N slots, if the burst is divided by up to j slots and N has a remainder when divided by j, the last two FEC blocks are of a smaller size than j. j is determined by an MCS according to the standard. Table 1 below lists values of j for CTCs.
TABLE 1MCSjBytes per slotQPSK, ½106QPSK, ¾6916QAM, ½51216QAM, ¾31864QAM, ½31864QAM, ⅔22464QAM, ¾22764QAM, ⅚230
The information sequence of a data burst takes the form of MAC Protocol Data Units (PDUs). A MAC PDU is for processing MAC-layer data. Its payload results from fragmentation or concatenation of MAC Service Data Units (SDUs).
FIG. 3 illustrates the structure of a MAC PDU in the IEEE 802.16e communication system.
Referring to FIG. 3, a Generic MAC Header (GMH) 310 is followed by payload 320 and a Cyclic Redundancy Check (CRC) 330. The GMH 310 and the CRC 330 are used for distinguishing and processing data on a MAC PDU basis in the MAC layer.
FIG. 4 illustrates the structure of the GMH in the IEEE 802.16e communication system.
Referring to FIG. 4, Header Type (HT) indicates the type of a header. That is, the HT indicates whether this header is a GMH or any other header. Encryption Control (EC) indicates whether payload is encrypted or not. CRC Indicator (CI) indicates whether the payload is followed by a CRC. Encryption Key Sequence (EKS) provides an encryption key. CID defines the connection that this PDU is servicing. Header Check Sequence (HCS) represents a CRC for the GMH. In the IEEE 802.16e communication system, there are two types of CIDs, basic CID and transport CID. The basic CID is specific to each Mobile Station (MS) and the transport CID is a CID related to MAC-layer data, identifying a connected service.
In the IEEE 802.16e communication system, information about data bursts is found in the DL-MAP. The DL-MAP provides information about a frame structure and information about all bursts included in a frame. Especially, information about the data bursts are provided by information providing units called Information Elements (IEs). A burst IE provides a basic CID specific to a particular MS so that the MS only can decode a burst corresponding to the burst IE. If the DL-MAP does not provide CIDs associated with the data bursts to reduce its overhead, every MS should decode the data bursts in the physical layer.
FIG. 5 illustrates the structure of a data burst in the IEEE 802.16e communication system.
Referring to FIG. 5, data with one or more PDUs is fragmented into FEC blocks in the physical layer according to a fragmentation and concatenation rule in a PDU domain 530. These FEC blocks are called FEC information blocks 520.
The FEC information blocks 520 are encoded to codewords. These codewords are called FEC codeword blocks 510. The FEC codeword blocks 510 are concatenated, thus producing a coded burst. The coded burst is Quadrature Amplitude Modulation (QAM)-modulated and allocated to frequency resources according to an allocation rule.
FIG. 6 is a block diagram of a decoder controlling apparatus in a receiver in the IEEE 802.16e communication system.
Referring to FIG. 6, the decoder controlling apparatus includes a soft metric buffer 610, at least one CTC decoder 624, at least one decoder input buffer 622, at least one decoder output buffer 626, a burst memory 630, and a controller 620.
The soft metric buffer 610 has FEC soft metric blocks. The size of an FEC soft metric block is equal to a maximum codeword size defined by the standard. The FEC soft metric block size is 960 in the IEEE 802.16e communication system. If one soft metric is represented in Z bits, the FEC soft metric block has 960Z bits.
FEC blocks that form a burst can be allocated to the FEC soft metric blocks. If one or more FEC soft metric blocks are completely stored in the soft metric buffer 610, decoding starts.
The FEC soft metric blocks are moved to the decoder input buffers 622. When the FEC soft metric blocks are completely stored in the decoder input buffers 622, the CTC decoders 624 CTC-decode the stored FEC soft metric blocks and then store them in the decoder output buffers 626. The decoded data are FEC information blocks. The FEC information blocks are rearranged at predetermined positions of the burst memory 630 on a burst basis. In general, the FEC information blocks that form one burst are controlled to be sequentially stored. The decoder controller 620 controls the FEC information blocks stored in the decoder output buffers 626 to be transferred to appropriate positions in the burst memory 630. When one burst is completely stored in the burst memory 630, the burst is provided to a MAC processor of the receiver.
As described above, MAC PDUs from various services can be transmitted in one burst in the IEEE 802.16e communication system. Especially when a DL-MAP IE does not have CID information about a burst, every MS should decode the burst.
For example, if during communications between an MS and a BS, a 20-Mbps burst is allocated without CID information and then a 3-Mbps burst is allocated to the MS, the MS should decode the bursts of 23 Mbps even though the 20-Mbps burst has no data for the MS, because there is no way to distinguish them. As a result, power consumption is increased in the MS.
Moreover, if the decoding capacity of the MS is 10 Mbps, the MS may not decode the 3-Mbps burst due to the preceding 20-Mbps burst.
Accordingly, there is a need for an improved decoding apparatus and method that decodes only necessary FEC blocks, thus saving power consumption.